Medical devices have been used to deliver therapeutic agents locally to the body tissue of a patient. For example, stents having a coating containing a therapeutic agent, such as an anti-restenosis agent, can be effective in treating or preventing restenosis. Currently, such medical device coatings include a therapeutic agent alone or a combination of a therapeutic agent and a polymer. Both of these types of coatings suffer from certain limitations.
Coatings containing a therapeutic agent without a polymer are generally impractical since such coatings offer little or no control over the rate of release of the therapeutic agent. Therefore, many medical device coatings include a therapeutic agent and a polymer.
Though the use of polymers can provide control over the rate of release of the therapeutic agent, the use of such polymers in coatings may pose certain other limitations. For example, some polymer coating compositions do not actually adhere to the surface of the medical device. In order to ensure that the coating compositions remain on the surface, the area of the medical device that is coated, such as a stent strut, is encapsulated with the coating composition. However, since the polymer does not adhere to the medical device, the coating composition is susceptible to deformation and damage during loading, deployment and implantation of the medical device. Any damage to the polymer coating may alter the therapeutic agent release profile and can lead to an undesirable increase or decrease in the therapeutic agent release rate. Also, polymer in the coatings may react with the blood and cause late stage thrombosis.
For instance, balloon expandable stents must be put in an unexpanded or “crimped” state before being delivered to a body lumen. During the crimping process coated stent struts are placed in contact with each other and can possibly adhere to each other. When the stent is expanded or uncrimped, the coating on the struts that have adhered to each other can be damaged, torn-off or otherwise removed. Moreover, if the polymer coating is applied to the inner surface of the stent, it may stick or adhere to the balloon used to expand the stent when the balloon contacts the inner surface of the stent during expansion. Such adherence to the balloon may prevent a successful deployment of the medical device.
Similar to balloon-expandable stents, polymer coatings on self-expanding stents can also interfere with the delivery of the stent. Self-expanding stents are usually delivered using a pull-back sheath system. When the system is activated to deliver the stent, the sheath is pulled back, exposing the stent and allowing the stent to expand itself. As the sheath is pulled back it slides over the outer surface of the stent. Polymer coatings located on the outer or abluminal surface of the stent can adhere to the sheath as it is being pulled back and disrupt the delivery of the stent.
An alternative to coating or encapsulating the surface of a medical device is to create pores within the surface of the medical device and dispose a therapeutic agent within the pores. Though the use of a porous surface overcomes certain limitations of using a polymer coating, due to the small size of the pores the therapeutic agent may only penetrate to a certain depth of the porous coating. Such insufficient penetration can result in a limited amount of the therapeutic agent that can be loaded onto the medical device, as well as, an unwanted rate of release where the therapeutic agent is released over a short period of time. Also due to the limited surface area of the surface of the medical device, a limited number of pores and therefore, a limited amount of a therapeutic agent can be loaded onto the surface of the medical device.
Accordingly, there is a need for medical devices and coatings for medical devices that have little or no polymer and that can release an effective amount of a therapeutic agent in a controlled release manner while avoiding the disadvantages of current coatings for medical devices. Also, there is a need for coatings that can release an effective amount of a therapeutic agent in a controlled release manner that can be selectively applied to the surfaces of a medical device, such as the surfaces that contact the body tissue of a patient. Additionally, there is a need for methods of making such medical devices and coatings for medical devices.